In the production of non-woven fabrics, such as spunbond and meltblown fabrics, molten polymer is extruded through a many holed die, or spinneret. The fibers are drawn by an air current and then collected in a random array to form a web of fibers. The web thus formed is then bonded by heating it to soften the polymer fibers and then passing the web through smooth or engraved calendar rollers to bond the fibers. Bonding improves the strength of the non-woven fabric over non-bonded materials.
In many cases, the bonding process governs the speed at which a producer can run a line producing non-woven fabrics. The time that the web requires between the nips in the calendaring process is controlled by the properties of the polymer in the fibers that make up the web, as well as the temperature and pressure of the calendar rolls.
In certain applications it is desirable to combine one or more webs of non-woven fabric. Typical constructions of this type combine one or more webs of spunbond fabric with a web of meltblown fabric. Such constructions are termed spunbond-meltblown (S-M) and spunbond-meltblown-spunbond (S-M-S) fabrics. A problem arises in the bonding process for such constructions stemming from the relatively high temperatures needed to bond the spunbond material. These temperatures may cause partial melting of the much finer fibers that make up the meltblown web, resulting in pinholes which compromise the barrier properties of the fabric.
It is known that a PP having a high level of solubles can improve process performance in non-woven fabrics, providing lower bonding temperatures and a wider bonding window. However, as with the production of polypropylene for BOPP films, it is a burden to PP manufacturers to produce high solubles because large amounts of unwanted wide-spec materials are generated to reach targeted high solubles. Also, it is known that the solubles are responsible for smoke generation during fiber spinning.
BOPP (biaxially oriented polypropylene) film is produced by drawing a cast sheet of polypropylene in two directions at a temperature below the melting temperature of the resin. Specific characteristics are required for the standard polypropylenes used to produce BOPP materials, such as relatively larger amounts of xylene solubles, and relatively low isotacticity. It is known that for a given PP, the lower the isotacticity, the lower the melting temperature of the PP and the better its processability to BOPP film. However, these same properties in the PP result in poorer properties of the resulting film. Therefore, there exists a processability-property trade-off in BOPP materials. In addition, production of high solubles materials generally used for BOPP films is not easy because it requires a specific catalyst system and careful handling of powder. It is known that it is difficult to produce a homopolymer containing xylene solubles fractions higher than 6% because a specific catalyst system as well as careful handling of polymer powder in the reactor are required. In general, the large amounts of xylene solubles in the polypropylene become sticky and often cause agglomeration of polymer powder in the reactor, disrupting continuous production at the plant.
To avoid the problems of producing high solubles material, blends that improve the processability of low solubles material have been investigated for BOPP film. It is well known that isotactic PP (iPP) produced by a Ziegler-Natta (ZN) catalyst has a broad isotacticity and molecular weight distribution, thus exhibiting a broad melting temperature range. Conversely, PP produced by a metallocene catalyst exhibits narrow isotacticity and molecular weight distribution and thus, the melting temperature range is relatively narrow. Unlike PP produced by ZN catalyst, some degree of regio-mis-insertion, i.e., “head-to-head” or “tail-to-tail” insertions, of monomer exists in the metallocene isotactic PP (m-iPP). The melting temperature of m-iPP is also affected by the degree of regio-mis-insertion in addition to isotacticity. Thus, an iPP of much lower melting temperature than conventional ZN-iPP can be produced with a metallocene catalyst. When employed in BOPP film, however, a much narrower temperature window for drawing is available due to the narrow tacticity and molecular weight distribution.
The effect of the addition of m-iPP to ZN-iPP on BOPP film was explored by Phillips et al, J. of Applied Polymer Science, 80, 2400 (2001). It was found that the addition of m-iPP to ZN-iPP provides a balance of elevated temperature draw performance and room temperature film properties relative to the ZN-iPP materials. Improved processability of the BOPP film including fewer webs breaks and drawability at higher line speeds have been claimed by the addition of some amounts of metallocene syndiotactic PP to ZN-iPP in U.S. Pat. No. 6,207,093 to Hanyu, Mar. 27, 2001, Fina Technology. The addition of some amounts of modifier tends to improve processability of iPP and/or properties of the resulting film. The selection of the modifier depends on the desired film properties and availability of modifier.
In U.S. Pat. No. 5,691,043, to Keller et al addition of various atactic and syndiotactic polypropylenes, as well as various propylene copolymers to a standard BOPP grade isotactic polypropylene homopolymer to produce a core layer for multi-layer a uni-axially shrinkable film are discussed. However, Keller does not discuss the possibility of replacing standard BOPP grade polypropylene homopolymers with low soluble content material.
In addition to seeking replacements for high solubles polypropylenes, BOPP film manufacturers have long sought a material that provides a stiffer oriented film while maintaining acceptable stretchability. High crystalline PP materials impart the desired stiffness to the finished articles, however, these materials are generally not suitable for processing into BOPP films. This poor operability of high crystalline materials is reported in U.S. Pat. No. 5,691,043.
Previously in related U.S. Pat. No. 6,733,898 the inventors disclosed blends of highly crystalline propylene homopolymer with an ethylene/propylene random copolymer and BOPP films produced using those blends. Blends according to the invention demonstrated good processability in BOPP films and imparted desired characteristics to the finished film in terms of stiffness.
It would also be desirable to provide a polypropylene material for non-woven fabrics that provides the wide bonding window and low bonding temperatures of a high solubles material without the drawbacks of large amounts of off-spec material and increased smoke generation in processing. It would further be desirable to provide a polypropylene material for producing non-woven fabrics that avoids the problems associated with producing high soluble content PP homopolymers. It would further be desirable to provide a process for producing a blend of high crystalline polypropylene homopolymer with a propylene copolymer by in reactor blending.